Experts warn against UK vaccination strategy as Biden calls for ramping up deployment

The U.K. announced it will be delaying second doses up to 12 weeks.

"Bad idea," said Dr. Paul Offit, a member of the Food and Drug Administration's Vaccine Advisory Committee. "You're disrupting a program," he said, adding if a recipient's second dose is delayed "two months later, three months later, four months later ... they may not be protected anymore."

COVID-19 vaccine injections have lagged globally, with the U.S. having only inoculated over 5 million of the approximate 20 million vaccines distributed around the country. A spokesperson for Joe Biden said in a statement on Friday that the president-elect wants to deploy as many vaccines as possible as quickly as possible, a departure from the Operation Warp Speed plan to hold some back enough to make sure everyone who gets a first vaccination also gets a second.

"It's all we're seeing really, day in and day out -- more and more cases of this," said Dr. Laith Alexander of St. Thomas' Hospital in the U.K. Alexander received his first injection last month, and said he did not agree to have his second shot delayed.

"The problem is I consented to the vaccination as per the schedule that was tested in the trial -- in the phase three trial," said Alexander. "I didn't really consent to a schedule that ultimately doesn't have a randomized clinical trial behind it."

The authorized vaccines in the U.K. -- including those from Pfizer, AstraZeneca and Moderna -- each require two doses. Each vaccine has some data showing that the vaccine could be moderately effective after the first shot, but not nearly as much as they would be without the second required dose.

"With the Pfizer [first dose], it was about 50% effective or so within that three-week period of time," Offit told ABC News regarding the data from the vaccine clinical trials. "For the Moderna [first dose], about 80% within the four-week period of time -- but all that shows is that you have a very, very short range of protection with that first dose, which may only be a few weeks."

Offit is a part of the advisory committee that voted for the FDA to authorize both Pfizer's and Moderna's COVID-19 vaccine candidates for emergency use in the United States.

"That second dose gives you an excellent boost response as was shown in the clinical trials," said Offit. "It's making a mistake to just delay the second dose because it sends a subtle message that the second dose perhaps isn't so important -- whereas it's critically important because that is what gives you the protection, and likely longer-term protection, you need from these vaccines."

The World Health Organization also weighed in on the U.K.'s plan, saying scientific evidence does not support a delay of more than six weeks in administering a second vaccine dose.

Offit said there is no support among researchers at the FDA to delay second doses in the U.S. One option being discussed, however, is administering just half the volume of the Moderna vaccine when giving each dose, which is based on early results in 18- to 55-year-olds.

The idea was posed by Moncef Slaoui, the head of Operation Warp Speed, in an interview with CBS' "Face the Nation" earlier this week.

"That makes even less sense than giving one dose and waiting," Offit said when asked about Slaoui's proposal. "I think it's a dangerous, dangerous idea. And certainly the Food and Drug Administration has squelched both the ideas."

When asked to comment, Moderna said, "At this point we wouldn't have any further information to share about any potential ongoing regulatory discussions."

On Monday, the FDA issued a warning against any experimenting with dosing. "We want to remind the public about the importance of receiving COVID-19 vaccines according to how they've been authorized ... in order to safely receive the level of protection observed in the randomized clinical trials," read the statement.

injections in the U.S. continue to be slower than officials originally promised, but Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the U.S. may soon be able to vaccinate a million people per day.

Eyes remain on the ongoing clinical trials for Johnson and Johnson's single-dose vaccine candidate. If the company proves to the FDA its vaccine is safe and effective, injections could be expedited starting as soon as February with up to 100 million doses distributed in the U.S., according to the company.

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